Bai Sema

Bai Sema (Thai: ใบเสมา) are boundary stones which designate the sacred area for a phra ubosot (ordination hall) within a Thai Buddhist temple (wat); otherwise called sema hin (เสมาหิน).

It is written in the Vinaya Pitaka that the Buddhasima (the area in which the monks perform the Sanghkamma or Buddhist sacred ceremonies) must be able to accommodate 21 seated monks, with a space the length of a forearm between each monk. The area should also not be larger than three yojana which is about 48 kilometers. This is probably meant to mean the maximum circumference. In other words, the Buddhasima should not be too small, nor too big. It is also written that the Buddhasima can be marked by the following Nimitta (border markers): hills, rock formations, forests, trees, ant hills, streets, rivers and other waters such as the sea or a pond. The type of Nimitta that is mainly used in Thailand is a stone. The oldest stone Bai Sema in Thailand were found in northeast Thailand (Isan) and are from the Dvaravati period (6th - 9th century CE).

Before work starts on a new phra ubosot, nine holes are dug: eight at the cardinal points, the ninth beneath where the principal Buddha statue will be placed. Luk Nimit (Thai: ลูกนิมิต), round stones the size of a cannonball, are placed in to these holes during a religious ceremony. Eight Sema stones are then placed over those Luk Nimit which are situated at the cardinal points. Double (or even triple) Bai Sema signify that the phra ubosot has been rebuilt, or consecrated for use by more than one monastic order, or that the temple has a Royal connection (photo 1).

The Thai words Bai Sema mean "Sema leaves" due to the shape of the flat Sema stones being somewhat akin to the shape of the leaves of the Bodhi tree, the tree under which the Buddha achieved enlightenment (photo 1).

...
Wikipedia